This invention relates to a.c. rotary dynamoelectric machinery and more particularly to such machines which may be used as rotary phase converters.
Rotary converters convert single-phase a.c. utility power to three-phase power. They therefore have many applications in farm and industrial settings where three-phase electrical machinery such as motors are needed, but where three-phase utility power is unavailable. In use, such converters may supply three-phase power to one or many electrical machines and ideally a rotary converter is capable of supplying this power indefinitely so long as the electrical load does not exceed the rated capacity of the converter.
A major problem with rotary converters is that as load conditions vary, electrical imbalances are created which result in unbalanced currents in the three-phase load. Depending upon the degree of this unbalance, excessive currents may exist in one or more of the phases of the load. If the load is, for example, a three-phase motor, a current may be supplied to one of the motor windings which exceeds the maximum rated current carrying capacity for the winding and this causes overheating, particularly of the winding insulation. As a rule of thumb, the useful like of such an electrical machine is effectively halved for every 10.degree. C. of overheating to which the insulation is subjected, necessitating more frequent machine replacement and increased expense to the owner or operator of a facility in which the machine is used.
There are presently two approaches to solving this problem. First, the rotary converter may be derated so that it supplies power to only a fraction of its rated load capacity. This, however, is no real solution since additional rotary converters are now required to service the three-phase equipment and the capital investment for the facility is increased. The second approach has been to add additional capacitance to the system, preferably on the load side of the converter. It has been found, however, that while this may improve the current balance somewhat, it is far from an adequate and economical solution to the problem since such additional capacitance increases both the cost and size of the converter.